Years ago there were process controls in which each step of the process had one or more loads (such as motors or valves), controlled by an array of switches or relays. These switches and loads were connected together in the conventional fashion to achieve the desired result. A disadvantage resulted in that each switch or relay contact often had to carry the full current to the load; therefore, the apparatus had continuous problems especially as the equipment grew older.
More recently electronic process controls have replaced the earlier controls. Some of these electronic process controls have controlled the load or loads based upon an analysis of a hypothetical ladder diagram comprising the input switches (or other circuit elements) and the loads. The switches were connected to an input multiplexer, and the loads were connected to the outputs of an output multiplexer, and the electronic process control analyzed the hypothetical ladder diagram, received input information from the input multiplexer, and controlled the output multiplexer to achieve the desired control over the loads. Three such electronic process control systems are described in Chapter 2 of the doctorate thesis of co-inventor Alfred Charles Weaver entitled "A Graphically-Programmed, Microprocessor-Based Industrial Controller", submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Computer Science in the Graduate College of the University of Illinois at Urbana-Campaign, 1976, Urbana, Illinois. Moreover, one of these prior electronic processors is shown and described in U.S. Pat. No. 3,761,882 to Peter G. Bartlett, Donald E. Henry, and Thomas Murrell, entitled "Process Control Computer", dated Sept. 25, 1973.
However, the aforesaid prior machines leave something to be desired. They are too complex for many applications and industrial engineers have found difficulties in putting the same to use. Moreover, they are unduly expensive for many applications.
It is, therefore, an object of this invention to provide a process control system for solving a hypothetical ladder diagram that is lower in cost than those previously available.
Another object of the invention is to provide a process control system for solving a hypothetical ladder diagram that is simpler in construction than those previously available.
Still another object of the invention is to provide a process control system, capable of solving a hypothetical ladder diagram that is more reliable in operation than the prior ones.
Still another object of the invention is to provide a process control system that is simpler to put into use by persons unskilled in the technology involved.